Generally speaking, there is a desire for illumination devices that are capable of generating light with a variable light intensity (dimming) and variable color. As should be clear to a person skilled in the art and therefore needs no elaborate explanation, it is possible to generate light of all possible colors in a large portion of the color gamut with a system that comprises three LEDs generating light of mutually different colors. In a typical example, one LED generates RED light, a second LED generates GREEN light, and a third LED generates BLUE light. The combined light output of these three LEDs has a mixed color within the color triangle defined by the colors of these three LEDs, and the exact color point within this color triangle depends on the mutual ratios of the intensities of the three LEDs. Thus, varying the color point of the system can be done by changing the relative intensity of one of the three LEDs, whereas varying the intensity of the light output while maintaining the color point can be done by changing the intensities of all LEDs to the same extent.
It is noted that it is possible to use more than three LEDs with mutually different colors; in such case, the present invention can also be applied, with suitable adaptations, as will become clear to a person skilled in the art.
For controlling the intensities of the respective LEDs, the system comprises a controller, typically implemented as a microcontroller. The microcontroller has an input for receiving a set signal, for instance from a central microcontroller or PC. The microcontroller further has three control outputs, one for each LED, for controlling the operation of the respective LEDs. Typically, the LEDs are operated with a variable duty cycle to achieve variation of the respective light intensities. The control output signals from the microcontroller to the respective LEDs are generated on the basis of the received input set signal, and on the basis of formulas or tables stored in a memory and defining a one-to-one relationship between input set signal and set points of the respective LEDs.
A problem in this respect is the fact that, even when controlled by a constant control signal, the intensities and color (wavelength) of the LEDs may vary, for instance under the influence of changing temperature, or for instance as a result of ageing. A further aspect of the problem is that the individual LEDs are not necessarily affected to the same extent, so there is differential variation. As a consequence, the color point of the system may vary with temperature and time. In order to prevent such color point variation, the controller should be provided with some compensation mechanism.
Such compensation mechanisms for the controller are known per se. A first compensation mechanism is called “temperature feed forward”, for short TFF. The system is provided with temperature detecting means detecting the temperature of the LEDs, specifically the junction temperatures of the individual LEDs. The said memory contains formulas or tables for correcting the said one-to-one relationship on the basis of the measured temperature. In a possible embodiment, the said memory comprises a matrix of LED control tables as a function of temperature, and the controller uses the “correct” table corresponding to the current temperature. It is also possible that the said memory comprises a matrix of correction factors as a function of temperature, and the controller reads the control signals from the table on the basis of user setting and applies the correction factors on the basis of the current temperature. An advantage of this compensation mechanism is that it is relatively fast, but a drawback is that it relies on predetermined data and does not take into account possible deviations from the predetermined data. Further, a drawback is that this compensation mechanism can not compensate for variations caused by ageing.
A second compensation mechanism is based on feedback of the light output (“flux feedback”, for short FFB). The system is provided with an optical sensor for sensing the actual light output (flux) of the individual LEDs, and the controller adapts its drive signals such that the actual light output of the LEDs is equal to the intended light output. An advantage of this compensation mechanism is that it does not need to have data regarding temperature response determined in advance, and that it always takes into account the actual light output situation. However, a drawback of this compensation mechanism is that it requires three optical sensors, one per LED, thus adding to the hardware costs. To reduce this hardware problem, a variation of this compensation mechanism is known, where the system comprises only one common optical sensor for sensing the overall light of the combined light output of the LEDs. This mechanism further requires a specific timing of the individual LEDs, to assure that it is possible to obtain measuring signals from which the individual light outputs can be derived, for instance by assuring that there are time intervals when only one of the LEDs is ON while all others are OFF. Now a disadvantage is that the flux measurement requires a minimum amount of time. This puts a restriction on the lower limit of the duty cycle that can be set for the LEDs, thus a limitation of the color points that can be set and a limitation of the dimming range.
It is noted that European patent 1.346.609 discloses a system where a controller comprises a TFF part and an FFB part operating in series, wherein the TFF part and the FFB part are active simultaneously. Although in such system the TFF part can compensate some of the disadvantages of the FFB part, the restriction on the lower limit of the duty cycle that can be set for the LEDs remains a problem caused by the FFB part.
It is an important objective of the present invention to overcome the above disadvantages.